1. Technical Field
The present invention relates to a piezoelectric device such as a piezoelectric vibrator or a piezoelectric oscillator used for various kinds of electronic equipment, and specifically, to a surface-mount piezoelectric device that air-tightly seals a package accommodating a piezoelectric vibrating reed.
2. Related Art
In related art, as piezoelectric devices, many surface-mount piezoelectric devices suitable for mounting on a circuit board or the like have been used. Generally, a surface-mount piezoelectric device includes a box-shaped thin base made of an insulating material such as ceramic or the like, and a flat plate-like lid joined thereto, and employs a package structure for mounting and air-tightly sealing a piezoelectric vibrating reed inside. Further, smaller and thinner piezoelectric devices are demanded as recent electric equipment becomes smaller and thinner.
To downsize the piezoelectric device, a tuning-fork piezoelectric vibrating reed having a pair of vibrating arms extending from a base end part in parallel, and a supporting arm extending from the base end part in parallel to the vibrating arms and provided with an extraction electrode from an excitation electrode is known (for example, see JP-A-2004-297198). A piezoelectric vibrating reed of thickness-shear mode having a vibrating part of a thin rectangular plate and a structure similarly including a supporting arm extending from a base end of the vibrating part in parallel thereto and provided with an extraction electrode from an excitation electrode is also known (for example, see JP-A-2009-21794).
These piezoelectric vibrating reeds are electrically connected and mechanically fixedly supported onto the supporting arm by fixing it to a mount electrode of the package with a conducting adhesive. Accordingly, compared to the related art structure in which the piezoelectric vibrating reed is cantilevered and fixedly supported with a conducting adhesive in the base end part, the dimension of the base end part in the longitudinal direction is smaller and the piezoelectric vibrating reed and the piezoelectric device may be downsized by the amount.
Typically, one supporting arm of the piezoelectric vibrating reed is provided for each side of the vibrating arm or the vibrating part for fixing the piezoelectric vibrating reed with balance. Further, only one supporting arm may be provided at one side of the vibrating arm or the vibrating part and fixed to the package in the supporting arm and the base end part (for example, see JP-A-2004-343541).
Furthermore, regarding the tuning-fork piezoelectric vibrating reed, the entire vibrating reed may be downsized in the longitudinal direction by forming weight parts having enlarged widths at the leading ends of the vibrating arms to shorten the vibrating arms (for example, see JP-A-2005-5896). By the wider weight parts, generation of the higher-order vibration may be suppressed and stability of the vibration frequency may be obtained.
In addition, in the surface-mount piezoelectric device, in order to eject gas and moisture produced from a sealing material at joining between the lid and the base and/or to seal the interior under a desired vacuum condition or in a desired atmosphere, a package structure in which a sealing hole communicating with the outside is provided in advance and the hole is closed after joining between the lid and the base is used. In many cases, the sealing hole of the package is provided in the base having a structure in which thin plates of a ceramic material are laminated (for example, see JP-A-2009-21794, JP-A-2004-343541, JP-A-2006-332727), however, it may be provided in the flat plate-like lid (for example, see JP-A-2002-171152).
The sealing hole of the package is closed by irradiating a spherical-shaped or pellet-shaped sealing material of a low-melting-point metal placed therein with a laser beam, a halogen lamp, or the like to instantaneously heating and melting the material. In this regard, it is preferable that the sealing hole has an inner opening diameter made smaller than an outer opening diameter so that the sealing material may not drop into the package. If its base has a laminated structure of ceramic thin plates, the sealing hole is formed to have a two-layer structure with a step by coaxially arranging a larger diameter hole provided in the ceramic thin plate forming the bottom plate and a smaller diameter hole provided in the ceramic thin plate laminated thereon. Further, in the lid of a metal flat plate, a sealing hole may be formed by forming a downward convex part using a jig such as a punch and opening a through hole at the center thereof.
The above described sealing hole in related art is provided to overlap with the piezoelectric vibrating reed in a plan view to reduce the plan dimension of the package in either case where the hole is provided in the base or the lid of the package. Particularly, in the case where the sealing hole is provided in a position overlapping with the excitation electrode of the piezoelectric vibrating reed, the vibration frequency of the piezoelectric vibrating reed may be fine-tuned by externally applying a laser beam or the like from the outside via the sealing hole to partially remove the excitation electrode after joining of the lid to the base.
However, in the package in which the sealing hole is provided in the base and arranged to overlap with the piezoelectric vibrating reed as described above, it is necessary that the bottom plate part of the base has a laminated structure with at least two layers. Further, a gap should be provided between the inner opening of the sealing hole and the piezoelectric vibrating reed. Accordingly, it is difficult to make the package thinner and shorter in height. Similarly, in the case where the sealing hole is provided in the lid, a gap should be provided between the convex part of the lid inner surface and the piezoelectric vibrating reed, and thinning and shortening of the package are limited.